Figure 12-16.Radial drill press.
After operating a drill press, wipe off all dirt, oil,
and metal particles. Inspect the V-belt to make sure no
metal chips are embedded in the driving surfaces.
Common drill bits are known as TWIST DRILLS
because most of them are made by forging or milling
rough flutes and then twisting them to a spiral
configuration. After twisting, the drill bits are milled to
the desired size and heat-treated.
The general-purpose twist drill is made of high-
speed steel. Figure 12-17 shows a typical plastic-cutting
drill bit and a typical metal-cutting drill bit. Notice the
smaller angle on the drill bit used for drilling plastics.
Drill bit sizes are indicated in three ways: by
inches, by letter, and by number. The nominal inch
sizes run from 1/16 inch to 4 inches or larger. The letter
sizes run from A to Z (0.234 inch to 0.413 inch).
Figure 12-17.Comparison of a twist drill for
plastic and a twist drill for metal.
The number sizes run from No. 80 to No. 1 (0.0135 inch
to 0.228 inch).
Before putting a drill bit away, wipe it clean and
then give it a light coating of oil. Do not leave drill bits
in a place where they maybe dropped or where heavy
objects may fall on them. Do not place drill bits where
they will rub against each other.
A drill bit should be reground at the first sign of
dullness. The increased load that dullness imposes on
the cutting edges may cause a drill bit to break.
When drilling steel and wrought iron, use a cutting
oil. Cast iron, aluminum brass, and other metals may be
drilled dry; therefore, at high-drilling speeds it is
advisable to use some medium for cooling these metals
to lessen the chances of overheating the drill bit with
the resultant loss of the cutting edge. Compressed air
may be used for cast iron; kerosene for aluminum; oleic
acid for cooper; sulphurized mineral oil for Monel metal;
and water, lard, or soluble oil and soda water for
ferrous metals. (Soda water reduces heat, overcomes
rust, and improves the finish.)
Sharpening Drill Bits
A drill bit becomes dull with use and must be
resharpened. Continued use of a dull drill bit may cause
it to break or bum up as it is forced into the metal.
Improper sharpening will cause the same difficulties.
Remove the entire point if it is badly worn or if the
margins are burned or worn off near the point. If, by
accident, the drill bit becomes overheated during
grinding, do NOT plunge it into the water to cool. Allow
it to cool in still air. The shock of sudden cooling may
cause it to crack.